The plasma-producing gases or mixtures of gases formed by argon-hydrogen or nitrogen, and possibly by the three constituents, have been known practically from the origin of the process of plasma arc cutting.
Since 1962, pure oxygen and compressed air have been proposed for the market in North America as plasma-producing gases in plasma cutting torches. However, for technical reasons such as insufficient knowledge of the process, of the equipment and of the behaviour of the electrodes, and for commercial reasons, the use of these plasma-producing gases which were more particularly recommended for mild steel have not had the expected immediate development.
The first publication relating to plasma cutting experiments carried out in the laboratory with nitrogen-oxygen mixtures in variable proportions different from air is due to the works of O'Brien described during the annual meeting of the American Welding Society, Detroit (Mich.) May 4 to 8, 1964. The author showed the advantages of an enrichment with oxygen by providing several curves of speed relative to the thickness to be cut and to the intensity of the arc. He also gave information relating to the reduction in the life of the electrodes when this percentage is increased. In this study, the author concluded that the nitrogen-oxygen mixture 80/20%, i.e. compressed air, is the best compromise.
Later, around 1970, the researchers of the Institut Von Ardenne, and then many other technicians constructed equipment employing either compressed air or pure oxygen to the exclusion of any other mixture.
It is well known that the life of electrodes used in compressed air plasma cutting, though short, nonetheless remains longer than that of the nozzles which are much more subjected to of the erosion of the arc, to the striking of the arc, to metal projections which can obstruct the nozzle, and even to handling incidents such as contact of the nozzle to the metal being cut. Experience in the art shows that users who must replace the worn nozzle also change the electrode for precautionary reasons so as to resume the work with a new electrode-nozzle "pair" and vice versa.
For example, with compressed air, or a 20/21% mixture of nitrogen and oxygen corresponding to the best proposed compromise, the life of the nozzle is approximately one half of the life of the electrode.